US Fish and Wildlife Service Verbesina Eradication Project
[This article summarizes information from Eastern Island, Midway Atoll National Wildlife Refuge Verbesina Eradication Update: November 26, 2012 by J. Klavitter, S. Schulmeister, P. Leary, and G. Schubert. U.S. Fish and Wildlife Service, Honolulu, HI. ]
Midway Atoll National Wildlife Refuge (NWR) is part of the Papahānaumokuākea Marine National Monument and one of the world's most spectacular national wildlife refuges. Formerly a Naval Air Station, each year, Midway Atoll is home to nearly three million nesting birds, including the endangered short-tailed albatross (Phoebastria albatrus), the endangered Laysan duck (Anas laysanensis), and the largest populations of Laysan albatross (Phoebastria immutabilis) or “gooney birds”.
“Midway Atoll is an amazing group of islands supporting and abundance of wildlife, but unfortunately humans have disturbed this fragile ecosystem,” explains Greg Schubert, Biological Sciences Technician with the U.S. Fish and Wildlife Service (USFWS) and field team leader for the invasive plant management program. “Our task is to enhance the habitat on the Atoll for all of our nesting seabird species as well as other biological resources.” A large part of the restoration efforts include controlling invasive plants and replanting specific areas with native or desirable vegetation.
Verbesina or golden crownbeard (Verbesina encelioides), is an invasive weed of the aster family that has modified the habitat of native plants, invertebrates, and ground-nesting birds, particularly the Laysan albatrosses on Midway Atoll NWR. The weed was likely introduced to Midway in seed-contaminated soil or equipment, or was possibly introduced as an ornamental. “There is evidence to suggest that the weed was introduced in contaminated soil as early as the 1930s,” explains Schubert. “Once it was established, it spread rapidly throughout Midway Atoll during war-time and post-war construction. However, landscaping during that time period would have kept the infestation from becoming apparent.” Although verbesina is the primary target for control, other invasive plants on Midway Atoll include perennial pepperweed (Lepidum virginicum), perennial sowthistle (Sonchus oleraceus), among several other herbaceous and woody vegetation.
IMPACTS TO THE BIRDS INCLUDE
Lower nesting density, reproductive success, and population declines. Thick stands of verbesina reduce both the quality and amount of nesting habitat.
Increased chick mortality. Verbesina grows very tall and dense which causes adults to expend more energy trying to locate their chicks for feeding. There is less airflow and thus higher temperatures in thick verbesina making chicks more susceptible to dehydration. The dense stands make it more difficult for the chicks to reach the ocean and fledge.
Egg and chick depredation from ants. Verbesina tends to harbor scale and aphid insects as well as ants. The ants irritate nesting adults and may prey on their eggs and chicks.
The USFWS’s verbesina management program on Eastern Island, Midway Atoll NWR began in the late 1990s and strengthened in 2003. The invasive plant management field team of six individuals is currently tasked with managing verbesina and other non-native plants on both Sand and Eastern Island (Figure 1).
“Year-round seabird nesting requires us to conduct our work without the use of heavy equipment or tractors. Control is accomplished mainly by crews applying herbicide with backpack sprayers, and to a lesser extent by hand-pulling,” explains Schubert. “Removing verbesina seeds from shoes, clothing, and equipment helps prevent reinfestation of the weed back into control areas." Treatments focus on controlling verbesina in management zones on Spit Island, Sand Island, and Eastern Island (Figure 1), and limiting ground disturbance caused by other management activities. “Verbesina seed remain viable in soil for several years, so any disturbance—including pulling or digging—can increase reinvasion of a site” says Schubert. “We also found that mowing was spreading verbesina seed to new locations and impacting ground nesting birds, so we no longer mow. Only weed wackers are used sparingly along roadsides near the residences in the central portion of Sand Island.” Crews also propagate native plants to restore areas where verbesina and other invasive plants have been controlled (Figure 2).
Initial herbicide treatments with glyphosate (Aquamaster®) produced satisfactory results. Finally, in July 2011, Refuge managers initiated experimental field trials with Milestone® specialty herbicide to determine the effectiveness of the herbicide for controlling verbesina. Milestone was applied as a highly diluted formulation of 5 cubic centimeters (5 milliliters) per 2.5 gallons (10 liters) of water (0.05% v/v). Two weeks after treatment, Milestone effectively controlled verbesina, while desirable native bunch grass remained vigorous (Figure 3).
“Our crews are sold on Milestone for verbesina removal since the herbicide effectively controls the plant and retreatment of the area is not needed for a minimum of eight weeks compared to three weeks or less control with glyphosate. In addition, desirable grasses are not affected. One control site on Eastern Island did not need retreatment for 11 months following Milestone application,” explains Schubert. The timing of retreatment varies somewhat with the season, but longer-term control of verbesina with Milestone allows crews to expand control efforts rather than continually retreat the same area. Crews also use much less active ingredient with Milestone® and apply less often, so total amount of herbicide used is less compared to glyphosate (Figure 4).
Ideal habitat for Laysan albatross includes open areas with bunchgrasses for nest building (Figure 5 and 6). The native bunchgrass Akiaki grass (Sporobolus virginicus) was present on the Atolls in the 1930s, but construction activities in the early 1940s may have caused its destruction. Restoration of weed-infested areas is performed with a combination of native and desirable grasses and plants that are grown in a nursery on Sand Island (Figure 2). Schubert explains that bunchgrass (Eragrostis variabilis) and the native Akiaki grass are both planted during restoration. Bunchgrass is planted along roadsides and trails to reduce verbesina invasion, and seashore dropseed is used for dune stabilization.
Monitoring data shows that the integrated control program is effectively reducing verbesina on Eastern Island. Treated areas have three percent or less aerial coverage of verbesina compared to 34 to 80 percent coverage in sectors not currently managed (Figure 7a and 7b). The removal of verbesina allowed a desirable native plant, native puncture vine (Tribulous cistoides), to restore the area naturally in some locations.
The USFWS invasive plant management team is committed to enhancing the quality of habitat on the Atoll for nesting seabirds and native plants. “We are excited about the early success of our efforts to eradicate verbesina and other invasive plants from Eastern, Sand and Spit Islands at Midway Atoll NWR. We have improved more than 178 acres of habitat and are gaining ground with new management tools,” Schubert concludes.
This project was funded by the U.S. Fish and Wildlife Service’s National Office and the National Fish and Wildlife Foundation and exemplifies a successful public-private partnership to restore a Pacific atoll ecosystem.
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